In Nov. 2008, just as the subprime-mortgage bust in the U.S. was driving the world financial system into meltdown, European Union finance ministers had an idea: Let’s add real-estate prices to the list of indicators we use to assess the economic situation in the euro zone.

Four years and two months later, Eurostat, the EU’s statistics agency, finally released its first EU-wide house-price index on Thursday. The headline result was somewhat underwhelming. Residential-property prices in the euro zone were down 2.5% in the third quarter compared with a year earlier. (Despite the name, the house-price index also includes apartments and other residential properties, as well as the land they are built on.)

The country-by-country statistics were more interesting. The biggest annual drop was observed in Spain, were house prices plummeted 15.2% from a year earlier and 3.7% from the previous quarter. More worryingly, the pace of decline shows little signs of slowing.

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Home owners in Ireland, meanwhile, may get to take a small breath on the steep downward slide they’ve been on for the past years; house prices there actually grew 1.6% compared to the second quarter, although they were still down 9.6% from a year earlier.

Things are also not looking good in the Netherlands—one of the countries that has often stood out lecturing others about their finances. Dutch house prices dropped 8.7% from a year earlier, the third-steepest fall in the euro zone after Spain and Ireland and the pace of decline seems to be getting faster by the quarter.

Given such dramatic drops—and increases of 8.4% and 7.1% in Estonia and Luxembourg, respectively—there’s little doubt that the new index will be useful for economic and monetary-policy decision makers as well as investors.

But why did it take so long to produce a usable index?

“It’s not a trivial matter to develop [data for] house prices,” says Laurs Nørlund, Eurostat’s national and European accounts director. Many euro-zone countries haven’t been systematically collecting house prices and even when they did, their data was often based on what real-estate agents and other appraisers said a home was worth, rather than what it actually sold for.

So even though European institutions like the European Central Bank, had some data on national real-estate prices, they weren’t as reliable or comparable across countries and thus not widely distributed.

Eurostat now has two staff working full-time on the house-price index, collecting national data based on information from land registries, mortgage providers, real-estate agents and construction companies.

“Now we are convinced that the quality of the data is such that we can publish it,” says Mr. Nørlund.

But even in Thursday’s release, there were some pretty obvious gaps. There was no data for Cyprus and Greece, while those for Austria and Germany were several quarters old.

“These holes in the table will be filled out,” promised Mr. Nørlund. For starters, new EU legislation that comes into force Friday will provide the legal basis for compiling new data that some of the hold-outs were missing, he added.

About Real Time Brussels

The Wall Street Journal’s Brussels blog is produced by the Brussels bureau of The Wall Street Journal and Dow Jones Newswires. The bureau has been headed since 2009 by Stephen Fidler, who was previously a correspondent and editor for the Financial Times and Reuters. Also posting regularly: Matthew Dalton, Viktoria Dendrinou, Tom Fairless, Naftali Bendavid, Laurence Norman, Gabriele Steinhauser and Valentina Pop.